This technology pertains to feedback reporting and feedback processing for spatial multiplexing schemes found for example in radio communications.
In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS) or “eNodeB” (LTE). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
In some versions of the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a radio network controller (RNC). The radio network controller, also sometimes termed a base station controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM). UTRAN is essentially a radio access network using wideband code division multiple access for user equipment units (UEs).
In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based radio access network technologies. Specifications for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) are ongoing within the 3rd Generation Partnership Project (3GPP). The Evolved Universal Terrestrial Radio Access Network (E-UTRAN) comprises the Long Term Evolution (LTE) and System Architecture Evolution (SAE).
Long Term Evolution (LTE) is a variant of a 3GPP radio access technology wherein the radio base station nodes are connected to a core network (via Access Gateways, or AGWs) rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes (eNodeB's in LTE) and AGWs. As such, the radio access network (RAN) of an LTE system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller (RNC) nodes.
Wireless communication over channels having multiple transmit and multiple receive antennas has generated a great deal of interest over the last decade. Multiple-input and multiple-output (MIMO) is the use of multiple antennas at both transmitter and receiver to improve communication performance. Multi-antenna techniques can significantly increase the data rates and reliability of a wireless communication system. The performance is in particular improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a multiple-input multiple-output (MIMO) communication channel and such systems and/or related techniques are commonly referred to as MIMO.
A core component in the LTE standard is the support of MIMO antenna deployments and MIMO related techniques. One of the features in LTE Release-8 is the support of a spatial multiplexing scheme with possibly channel dependent precoding (see Love, D. J, Heath, R. W., Jr., “Limited feedback unitary precoding for spatial multiplexing systems”, IEEE Transactions on Information Theory, vol. 51, issue 8, pp. 2967-2976, August 2005, the disclosure of which is incorporated herein by reference.) The spatial multiplexing scheme is targeted for high data rates in favorable channel conditions. An example illustration of the spatial multiplexing scheme is provided in FIG. 1.
As seen in FIG. 1, the information carrying symbol vectors s are multiplied by an NT×r precoder matrix WNT×r. The matrix is often chosen to match the characteristics of the NR×NT MIMO channel matrix H. Each of the r symbols in vector corresponds to a layer and r is referred to as the transmission rank. LTE uses orthogonal frequency division multiplexing (OFDM), and hence the received NR×1 vector yk for a certain time-frequency resource element indexed k, assuming no inter-cell interference, is modeled by:yk=HWNT×rsk+ek  (1)where ek is a noise vector obtained as realizations of a random process.
The precoder WNT×r may be selected from a predetermined and finite set of countable precoders known to both the eNodeB as well as the UE, a so-called codebook. This restricts the eNodeB in the choice of precoder and is usually coupled with feedback reporting from the UE which recommends a precoder to the eNodeB. Another alternative is to give the eNodeB complete freedom in determining the precoder, so-called non-codebook based precoding. By using dedicated pilots, also known as UE-specific reference signals (RS), the UE does not need to be aware of which precoder has been used in the transmission and, in contrast to codebook based precoding, there is no quantization effect. Combinations of codebook based and non-codebook based approaches are also possible. For example, the feedback reporting could be codebook based while the transmission is non-codebook based by the use of UE specific RS. The latter approach corresponds to current standardization efforts for Rel-10 of LTE.
As already mentioned, the UE may, based on channel measurements in the forward link, transmit recommendations to the base station of a suitable precoder to use, including recommended transmission rank. In the case of codebook based precoding, the UE may perform an exhaustive search over all precoders in the codebook to find the one which gives the best performance, e.g., predicted throughput, and then feed back an index pointing to the best precoder to the eNodeB. A single precoder that is supposed to cover a large bandwidth (wideband precoding) may be fed back. It may also be beneficial to match the frequency variations of the channel and instead feed back a frequency-selective precoding report, e.g., several precoders, one per subband.
Channel dependent precoding as above typically requires substantial signaling support, particularly for frequency-selective precoding. Not only is feedback signaling in the reverse link (i.e., from UE to eNodeB in LTE) needed, as mentioned previously, but typically also signaling in the forward link (i.e., from eNodeB to UE in LTE) is required to indicate which precoder was actually used in the forward link transmission since the forward link transmitter (i.e. eNodeB) might not be certain that it obtained a correct precoder report from the (forward link) receiver (i.e., UE).
The encoded bits originating from the same block of information bits are referred to as a “codeword”. This is also the terminology used in LTE to describe the output from a single HARQ process serving a particular transport block and comprises turbo encoding, rate matching, interleaving, etc. The codeword is then modulated and distributed over the antennas. Such a transformed codeword is often also referred to as a “codeword” when there is no risk of confusion.
It may make sense to transmit data from several codewords at once, also known as multi-codeword transmission. The first (modulated) codeword may, for instance, be mapped to the first two antennas and the second codeword to the two remaining antennas in a four transmit antenna system. In the above context of precoding, the codewords are mapped to layers instead of direct mapping to the physical antennas.
In the field of high rate multi-antenna transmission, one of the most important characteristics of the channel conditions is the so-called channel rank. Roughly speaking, the channel rank can vary from one up to the minimum number of transmit and receive antennas. Taking a 4×2 system as an example, i.e., a system with four antennas at the transmitter side and two antennas in receiver side, the maximum channel rank is two. The channel rank varies in time as the fast fading alters the channel coefficients. Moreover, it determines how many layers, and ultimately also how many codewords, can be successfully transmitted simultaneously. Hence, if the channel rank is one at the instant of transmission of two codewords mapping to two separate layers, there is a strong likelihood that the two signals corresponding to the codewords will interfere so much that both of the codewords will be erroneously detected at the receiver.
In conjunction with precoding, adapting the transmission to the channel rank involves using as many layers as the channel rank. In the simplest of cases, each layer would correspond to a particular antenna. But the number of codewords may differ from the number of layers, as in LTE. The issue then arises of how to map the codewords to the layers. Taking the current working assumption for the 4 transmit antenna case in LTE as an example, the maximum number of codewords is limited to two while up to four layers can be transmitted. A fixed rank dependent mapping according to FIG. 2 is used.
The design and relative placement of the antennas has a strong impact on the performance of the system. There are naturally many different possibilities. A natural constraint is to keep the total array size as small as possible while maintaining good performance. Co-polarized, closely spaced antennas tend to result in correlated fading, which simplifies achieving array gain via beamforming, but on the other hand reduces the chance of enjoying high rank transmissions that tend to prefer uncorrelated fading.
Another way of obtaining uncorrelated fading, and in fact also limit the interference between layers while keeping the size of the antenna array small, is to transmit on orthogonal polarizations by using a co-located and cross-polarized pair of antennas. FIG. 3 illustrates, by vertical lines, eight antennas, the two cross-polarized antennas of a pair being commonly illustrated by an “X” to account for the ±45 degree orientations of the polarizations. A combination of orthogonal and closely spaced antennas is a promising array setup for 4 and 8 transmit cases. As also depicted in FIG. 3, by using pairs of cross-polarized antennas close to each (in the order of 0.5-1 wavelengths), the size of the array is kept small while at least up to rank 2 transmissions is well gathered for by means of transmission on orthogonal polarizations while achieving array gain is facilitated by the small distance between the cross-poles.
In the particular example of FIG. 3 two Common Reference Signals (CRSs), e.g., CRS#1 and CRS#2, can be used for orthogonal polarizations so that channel estimation is facilitated at forward link receiver. But reference signals can, if available, of course also be mapped in other ways on to the antenna array. For example, if eight reference signals are available, they can each be connected to a separate antenna. In Rel-10 of LTE, this may be a common scenario since there will then be support of up to eight cell-specific antenna ports and their corresponding reference signals.
In case of conventional precoder feedback, the size of the codebook directly determines the amount of signaling overhead. It is therefore desirable to strive for as small codebook as possible. On the other hand, a small codebook usually implies lower performance. This problem becomes more pronounced as the number of transmit antennas increases due to the need of a larger codebook to cover the increase in number of degrees of freedom that can be exploited for transmission. The overhead is particularly large when frequency-selective precoding is employed and thus multiple precoders covering the bandwidth are fed back. Such kind of precoding is typically required to track the fading across frequency in order to ensure the transmitted signals add constructively on the receiver side and also orthogonalize the channel for good separation of the layers.
A consequence of a large codebook and/or frequency-selective precoding is also high computational complexity for the selection of precoder, which is carried out on the UE side for precoder feedback to be used for downlink transmissions or on the eNodeB side in case of precoded transmissions from UEs in the uplink or in case of non-codebook based precoding. There is a substantial and increasing amount of number crunching involved as the precoder matrices grow larger.
A codebook or precoder determination procedure may also perform poorly for a particular antenna array setup. Matching the properties of the precoder to the particular antenna setup is therefore important as it can maintain high performance while at the same time reduce the overhead. To increase the benefits of multi-rank transmission and reduce the requirements on the need of advanced receivers, orthogonalization of the channel plays a crucial role. However, with normal codebook based precoding as in present Release-8 LTE, the orthogonalization effect is almost negligible due to too few precoders in rank 2 for 2 Tx transmission and in rank 2, 3 and 4 for 4 Tx transmission. At the time, this was considered appropriate in order to maintain reasonable overhead numbers.